The present invention relates in general to pumping mechanisms for supplying pressurized fluid to hydraulically actuated systems. In particular, the present invention relates to a gear pump having an inlet port shut-off valve, referred to as a dry valve, which is prevented from closing when the fluid pressure delivered from the outlet port of the gear pump to such a hydraulic system is greater than a predetermined maximum safe magnitude. The present invention also relates to an improved means for providing a flow of lubricating and cooling fluid through the gear pump when the dry valve is closed and, thus, the gear pump is operated in the dry mode.
Gear pumps are well known in the art and typically include a pair of gears mounted upon respective shafts for rotation within a pump housing. The shafts are arranged such that the gears mesh within a pumping chamber disposed between an inlet port and an outlet port. One of the shafts is rotated by an external source of power so as to cause the two gears to rotate. In this manner, hydraulic fluid is drawn from a reservoir through the inlet port and is discharged at a relatively high pressure from the outlet port to the hydraulic system.
One common use for gear pumps of this type is on a refuse packing vehicle. Such a vehicle is typically driven by an internal combustion engine and includes one or more movable packing mechanisms which are hydraulically actuated. A gear pump can be connected to and driven by the internal combustion engine to generate a flow of pressurized fluid to operate the packing mechanisms. Because of its size and reliability, the gear pump is well suited to perform this function. Typically, however, such packing mechanisms are used only intermittently, requiring no flow of pressurized fluid for long periods of time. The internal combustion engine, on the other hand, is usually continuously operated. Thus, for this and other uses, some means must be provided for selectively interrupting the flow of pressurized fluid from the gear pump to the hydraulic system.
Several structures are known in the art for accomplishing this selective interruption. A first known structure includes a valve provided between the outlet port of a continuously driven gear pump and the packing mechanisms actuated thereby. The valve is selectively actuated to direct the flow of pressurized fluid from the outlet port of the gear pump either to the packing mechanisms or back to the reservoir. Thus, when the packing mechanisms are not to be utilized, the flow of pressurized fluid is diverted from the packing mechanisms and is returned to the reservoir. Unfortunately, this structure results in undesirable power losses, particularly at high engine speeds and, therefore, at high flow rates.
A second known structure includes a power take-off unit or a clutch provided as a connection between the internal combustion engine and the gear pump. The power take-off unit or clutch selectively makes and breaks the driving connection between the internal combustion engine and the gear pump. When the packing mechanisms are not to be utilized, the power take-off or clutch is disengaged so as to disable the gear pump. As a result, the flow of pressurized fluid to the packing mechanisms is interrupted. These structures, however, increase the cost and complexity of the overall refuse packing system, as well as introduce additional components which are subject to failure.
A third and more preferable structure for interrupting the flow of pressurized fluid from the gear pump to the packing mechanisms includes a dry valve. The dry valve is well known in the art and can simply be described as a shut-off valve disposed in the inlet port of the gear pump. When closed, the dry valve obstructs the flow of hydraulic fluid from the inlet port to the pumping chamber of the gear pump. Consequently, the flow of pressurized fluid to the packing mechanisms is interrupted, even though the gear pump is continued to be operated. When the gear pump is operated while the dry valve is closed, it is said to be operating in the dry mode. Typically, means are provided in the dry valve for permitting a relatively small amount of hydraulic fluid to flow into the pumping chamber even when the dry valve is closed. Such relatively small amount of fluid flow is necessary for lubricating and cooling the components of the gear pump while it is operated in the dry mode.
Several problems associated with the use of a dry valve in a gear pump are related to such flow of lubricating and cooling fluid. In such prior art gear pumps, a bleed valve must be provided between the gear pump and the packing mechanisms to prevent the flow of lubricating and cooling fluid from inadvertently operating the packing mechanisms. The bleed valve causes the relatively small amount of lubricating and cooling fluid to be returned back to the reservoir after being circulated through the gear pump. The use of such a bleed valve results in the undesirable loss of a portion of the pressurized fluid when the dry valve is opened. Thus, it would be desirable to provide a gear pump which avoids this problem.
A third problem associated with the use of a dry valve in a gear pump can occur if the dry valve is attempted to be closed while the pump is generating a flow of hydraulic fluid at a relatively high pressure. Closure of the dry valve during such relatively high pressure output can cause a highly unbalanced pressure condition to occur within the gear pump. Specifically, the pressure within the pumping chamber drops rapidly to a relatively low level when the dry valve is closed. However, the pressure at the outlet port remains relatively high for a short period of time following such closure. In most gear pumps, such outlet port pressure is fed back within the gear pump to urge a pair of opposed pressure plates into sealing engagement with the sides of the gears. During this period of high pressure unbalance, the pressure plates exert excessively large forces against the sides of the gears, resulting in premature wear and failure of the gear pump. Thus, it would be desirable to provide a means for preventing the closure of the dry valve while the gear pump is generating a flow of hydraulic fluid at a pressure which is greater than a predetermined maximum safe magnitude.